1. Field of the Invention
The invention relates generally to greenhouse construction, and more particularly to double-wall greenhouses having flexible plastic walls.
2. Description of the Prior Art
In the past greenhouses have used glass panes mounted in a metal framework. However, they are very expensive to build and operate and, thus, are only within the financial reach of commercial growers. Such greenhouses are also vulnerable to breakage and to formation of water condensation on the glass panes. Such condensation blocks the sunlight and drips from the window frames, causing damage to the plants. To reduce condensation and cut heating costs, greenhouses have been double glazed, but only at further increase in construction costs.
Such greenhouses are, therefore, very expensive for commercial growers, and far too expensive for the home greenhouse market. It would be preferable to have a greenhouse which is compact, constructed of inexpensive materials, easily assembled, and inexpensive to operate. More specifically, a greenhouse should have a low roofline yet provide adequate standing head room. It should be constructed of inexpensive insulative materials, and include a double glazing. Such glazing should be relatively invulnerable to breakage, should be inexpensive, and should be easily installed.
Several attempts have been made at improving greenhouses and bringing down their costs. One design uses a wooden, rather than metal frame, and substitutes transparent plastic panes for conventional glass. However, such a greenhouse is still expensive to construct. Plastic panes cost nearly as much as glass, and substantial labor and materals is required to assemble the frame and mount the individual panes.
In another variation, sheets of corrugated fiberglass are applied to the roof and walls of the wooden frame. This creates several other problems. The corrugated material is hard to seal along its edges and is difficult to fit around the internal joists and corners of the frame to obtain adequate double-wall construction. In addition, fiberglass material lacks adequate transparency for many greenhouse applications.
In a further variation, sheets of flexible plastic film are attached to the frame by lathe strips nailed to the frame members. However, the plastic film is easily torn by the nails and by corners at the joints in the frame. It is difficult to seal the plastic sheets adequately against the wooden frame. Internal bracing, such as roof joists and corners make it very difficult to double glaze such greenhouses. Finally, such internal bracing necessitates a high roofline on the greenhouse in order to obtain sufficient standing head room. It also interferes with sunlight reaching the plants within the greenhouse.
It has also proven difficult to stretch the plastic film sufficiently to obtain a wrinkle free fit. Various types of framing strips and bead strips have been suggested for securing the edges of film along opposite sides of a window opening. Framing strips commonly have a groove or channel for receiving a complementary glazing or bead strip to secure a margin of the flexible film within the channel. Such an arrangement is shown in my own prior patent, U.S. Pat. No. 3,991,806 and patents cited therein. However, such fasteners either fail to stretch the fabric sufficiently to remove the wrinkles when assembled, or are unable to hold the film under sufficient tension without the bead strips pulling from their grooves.
A second greenhouse design uses an inflated structure of transparent plastic film. Such a greenhouse has no frame and therefore avoids the head room and shade problems, but it cannot be double glazed. Furthermore, such a greenhouse is expensive to keep inflated and vulnerable to damage.
A third design employs bowed aluminum arches over which plastic film is stretched. In this design it has been very difficult to obtain an adequate air seal at the bottom where the plastic meets the ground and around the ends. More importantly, such a design cannot be double glazed since there was no way of attaching a second wall to the inside of the aluminum bows.